Asphalt offers outstanding binding and waterproofing characteristics. These physical attributes of asphalt have led to its widespread utilization in paving, roofing, and waterproofing applications. For instance, asphalt is used in manufacturing roofing shingles because it has the ability to bind sand, aggregate, and fillers to the roofing shingle while simultaneously providing excellent water barrier characteristics.
Naturally occurring asphalts have been used in various applications for hundreds of years. However, today almost all of the asphalt used in industrial applications is recovered from the refining of petroleum. Asphalt, or asphalt flux is essentially the residue that remains after gasoline, kerosene, diesel fuel, jet fuel, and other hydrocarbon fractions have been removed during the refining of crude oil. In other words, asphalt flux is the last cut from the crude oil refining process.
To meet performance standards and product specifications, asphalt flux that is recovered from refining operations is normally treated or processed to attain desired physical characteristics and to attain uniformity. For instance, asphalt that is employed in manufacturing roofing products has to be treated to meet the special requirements demanded in roofing applications. More specifically, in the roofing industry it is important to prevent asphaltic materials from flowing under conditions of high temperature such as those encountered during hot summers. In other words, the asphaltic materials used in roofing products should maintain a certain level of stiffness (hardness) at high temperatures. This increased level of stiffness is characterized by a reduced penetration, an increased viscosity, and an increased softening point.
To attain the required level of stiffness and increased softening point that is demanded in roofing applications the asphalt flux is typically oxidized. This is typically done via an air blowing process. In such air blowing techniques, an oxygen containing gas, such as air, is blown through the asphalt flux for a period of about 2 to about 8 hours while it is maintained at an elevated temperature which is typically within the range of 400° F. (204° C.) to 550° F. (288° C.). The air blowing process results in the stiffness and the softening point of the asphalt flux being significantly increased. This is highly desirable because ASTM D 3462-96 (Standard Specification for Asphalt Shingles Made from Glass Felt and Surfaced with Mineral Granules) requires roofing asphalt to have a softening point which is within the range of 190° F. (88° C.) to 235° F. (113° C.) and for the asphalt to exhibit a penetration at 77° F. (25° C.) of above 15 dmm (1 dmm =0.1 mm). In fact, it is typically desirable for asphalt used in roofing applications to have a penetration which is within the range of 15 dmm to 35 dmm in addition to a softening point which is within the range of 185° F. (85° C.) to 235° F. (113° C.).
In typical air blowing techniques the oxygen containing gas is introduced and distributed into the bottom 14 of an un-agitated blow still 15 through spargers 16. Once the oxygen containing gas (air) is in the system it travels up through the asphalt 17 and ultimately reaches the surface of the asphalt 8 at the top of the blow still as illustrated in FIG. 1. As the air travel through the asphalt from the bottom to the top of the blow still it is available to react with the asphalt flux being oxidized. The rate of chemical reactions occurring within the blow still is known to be limited by the diffusion of oxygen in the air bubbles traveling through the system. It is also known that mechanical agitation has a significant effect on the oxidation processing time by increasing the surface area of the air bubbles in the system. In any case, conventional asphalt oxidation techniques are currently mass transfer limited.
Air blowing has been used to increase the softening point and stiffness of asphalt since the early part of the twentieth century. For example, U.S. Pat. No. 2,179,208 describes a process wherein asphalt is air blown at a temperature of 300° F. (149° C.) to 500° F. (260° C.) in the absence of a catalyst for a period of 1 to 30 hours after which time a polymerization catalyst is added for an additional treatment period of 20 to 300 minutes at a temperature of 225° F. (107° C.) to 450° F. (232° C.). Over the years a wide variety of chemical agents have been used as air blowing catalysts. For instance, ferric chloride, FeCl.3 (see U.S. Pat. No. 1,782,186), phosphorous pentoxide, P2O5 (see U.S. Pat. No. 2,450,756), aluminum chloride, AlCl3 (see U.S. Pat. No. 2,200,914), boric acid (see U.S. Pat. No. 2,375,117), ferrous chloride, FeCl2, phosphoric acid, H3PO4 (see U.S. Pat. No. 4,338,137), copper sulfate CuSO, zinc chloride ZnCl2, phosphorous sesquesulfide, P4S3, phosphorous pentasulfide, P2S5, and phytic acid, C6H6O6(H2PO3)6 (see U.S. Pat. No. 4,584,023) have all been identified as being useful as air blowing catalysts.
Several patents describe the application of phosphoric mineral acids in modifying asphalt properties. For instance, U.S. Pat. No. 2,450,756 describes a process to make oxidized asphalts by air blowing petroleum hydrocarbon in the presence of a phosphorus catalyst, including phosphorus pentoxide, phosphorus sulfide, and red phosphorus. U.S. Pat. No. 2,762,755 describes a process of air blow asphaltic material in the presence of a small amount of phosphoric acid. U.S. Pat. No. 3,126,329 discloses a method of making blown asphalt through air blowing in the presence of a catalyst which is an anhydrous solution of 50 weight percent to 80 weight percent phosphorus pentoxide in 50 weight percent to 20 weight percent phosphoric acid having the general formula HmRnPO4.
All of the air blowing techniques described in the prior art share the common characteristic of both increasing the softening point and decreasing the penetration value of the asphalt flux treated. In other words, as the asphalt flux is air blown, its softening point increases and its penetration value decreases over the duration of the air blowing procedure. It has been the conventional practice to air blow asphalt flux for a period of time that is sufficient to attain the desired softening point and penetration value. However, in the case of some asphalt fluxes, air blowing to the desired softening point using conventional procedures results in a penetration value which is too low to be suitable for utilization in roofing applications. These asphalt fluxes are called “hard asphalt fluxes”. In other words, hard asphalt fluxes cannot be air blown using conventional procedures to a point where both the required softening point and penetration values are attained. Accordingly, today there is a need for a process that can be used to air blow hard asphalt flux to both a softening point which is within the range of 185° F. (85° C.) to 250° F. (121° C.) and a penetration value at 77° F. (25° C.) of above 15 dmm.